ZTS: deletes home directories in /export/home

In the cleanup for the privilege tests, an empty variable, empty because
the corresponding setup is skipped on Linux, results in /export/home
being deleted. This patch adds an assertion that the variable is not
empty, and causes the cleanup to be skipped on Linux as well.

Reviewed by: John Wren Kennedy <jwk404@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Signed-off-by: John Gallagher <john.gallagher@delphix.com>
External-issue: LX-1099
Closes #7615